1. Field of the Invention
The present invention relates to a steering system in a wheeled vehicle the front-wheel steering linkage of which is operatively interconnected to a rear-wheel steering linkage to steer the rear road wheels in response to steering operation of the front road wheels, and more particularly to a rear-wheel steering mechanism capable of controlling the steering angle ratio and direction of the rear road wheels relative to the front road wheels in accordancd with travel condition of the vehicle.
2. Discussion of the Background
Such a steering system as described above has been proposed in Japanese Patent Early Publication No. 62-163870, which steering system comprises a front-wheel steering linkage, a rear-wheel steering linkage including a lateral control shaft operatively connected at its opposite ends with a set of rear steerable road wheels, and a rear-wheel steering mechanism operatively connected to the front-wheel steering linkage for effecting axial displacement of the lateral control rod in response to operation of the front-wheel steering linkage. The rear-wheel steering mechanism includes a housing mounted on a vehicle body structure for axially slidably and rotatably supporting thereon the lateral control shaft, a first carrier body mounted within the housing for rotation about a vertical axis perpendicular to the central axis of the control shaft and arranged to be rotated in response to operation of the front-wheel steering linkage, a second carrier body mounted within the first carrier body for rotation about the central axis of the control shaft, a radial rod integral with the control shaft and being slidably and rotatably carried by the second carrier body, and an electric motor mounted on the housing for rotating the control shaft in accordance with travel speed of the vehicle.
In operation of the rear-wheel steering mechanism, the lateral control shaft is axially displaced by rotation of the first carrier body to steer the rear road wheels and is rotated by activation of the electric motor to control the steering angle ratio and direction of the rear road wheels relative to the front road wheels in accordance with travel speed of the vehicle. The steering system further comprises an electromagnetic clutch mechanism arranged to connect or disconnect the rear-wheel steering mechanism to and from the front-wheel steering linkage, and an electric control apparatus arranged to control the clutch mechanism and the electric motor in such a manner as to disengage the clutch mechanism and return the control shaft to a neutral position in the occurrence of unexpected trouble in the rear-wheel steering mechanism. Due to such arrangement of the clutch mechanism and the electric control apparatus, the rear-wheel steering mechanism becomes complicated in construction, and the steering system becomes costly. Moreover, the rear-wheel steering mechanism may not be controlled in the occurrence of some trouble in the clutch mechanism, the electric motor, the electric control apparatus and their associated parts.
In Japanese Patent Early Publication No. 60-85077, there is disclosed a steering system which comprises a rotary shaft extending longitudinally in a fore-and-aft direction of the vehicle to be rotated in response to steering operation of the front road wheels, a swingable rod connected at its front end with the rotary shaft by means of a universal joint, a crank mechanism actuated by a piston rod of a hydraulic power cylinder to move the rear end of the swingable rod upwards or downwards, and a joint member coupled with the swingable rod and connected at its opposite ends with a set of rear steerable road wheels through a pair of tie-rods. In operation of the steering system, the swingable rod is moved leftwards or rightwards in response to rotation of the rotary shaft to steer the rear road wheels through the joint member and tie-rods and is moved upwards or downwards by operation of the crank mechanism to control the steering direction of the rear road wheels relative to the front road wheels. The hydraulic power cylinder in the system is provided therein with a compression spring which is arranged to bias the piston rod toward a stroke end. In case the power cylinder may not be supplied with hydraulic fluid under pressure due to unexpected trouble in a hydraulic control apparatus, the piston rod is moved to and retained in the stroke end under the biasing force of the spring to retain the rear end of the swingable rod in the upward position so as to steer the rear road wheels in the same direction as the front road wheels. In such control of the rear road wheels, the turning ability of the vehicle is deteriorated during travel at a low speed.